Existing retro-reflectors are generally produced as moulded glass "cats eye" reflectors (a combination of a concave mirror and a convex lens) or from moulded (generally plastic) cube-corner reflectors.
Examples of such retro-reflectors are shown in the following patents:
U.S. Pat. No. 4,358,217--Jones PA0 U.S. Pat. No. 4,504,169--Jones PA0 GB No. 1,530,630--Ferro Corporation PA0 GB No. 2,075,094 A--Stone PA0 GB No. 2,121,463 A--Egan et al. PA0 GB No. 2,128,664 A--Takahashi et al. PA0 GB No. 2,190,123 A--Horne PA0 U.S. Pat. No. 3,409,344--Balint et al.
Moulding imperfections in the finished product and the time required in accurately tooling the sharp corners required for the "cube-corner" reflectors render them less than satisfactory as highway lane markers.
In addition, existing retro-reflectors used in highway lane markers are designed "in theory" to reflect an incident light ray directly back towards its source. In practice, manufacturing imperfections are such that the returned light is diffused (generally symmetrically) as a cone of light centered about the reflector-source axis, so that the driver of a vehicle will see only a small fraction of the light returned from a highway lane marker.